Thin film formation process

ABSTRACT

A process for thin film formation is provided which comprises a step of separation of a substrate constituted of a nonporous Si layer, a porous Si layer formed thereon, and a less porous Si layer formed further thereon into the nonporous Si layer and the less porous Si layer at the porous Si layer, wherein the separation is caused by projecting a laser beam through the side face of the substrate. From the separated substrate, an SOI substrate is prepared, and the non porous Si layer is recycled to the SOI substrate production process. This SOI substrate production process saves the consumption of the material and lowers the production cost. The substrates are separated definitely. A process for producing a photoelectric transducing apparatus such as solar cells with material saving and low cost is also provided in which the porous layer is separated definitely without strong adhesion between the substrate and a jig.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for forming a thin film foran SOI substrate, or photoelectric transducers such as solar cells andarea sensors.

2. Related Background Art

The integrated circuit formed on a SOI substrate (semiconductor oninsulator) has various advantages over integrated circuits formed on ausual silicon wafer, such as (1) ease of separation of a dielectricmaterial and potentiality of high integration, (2) high resistanceagainst radiation rays, (3) low floating capacity and high speedprocessing, (4) no welling process required, (5) capability of latch-upprevention, and (6) high speed and low energy consumption owing toformation of a thin complete depletion type field effect transistor.

For formation of a substrate of the SOI structure, methods are disclosedin U.S. Pat. No. 5,371,037, and T. Yonehara et al., Appl. Phys. Lett.,vol. 64, 2108 (1994). FIGS. 16A to 16E, and FIGS. 17A to 17D show theprocesses. In the drawings, the numerals 1 and 5 indicate a Si wafer, 2a nonporous Si layer, 3 a porous Si layer, 4 an epitaxial Si layer, 6 asingle crystalline Si layer, and 7 a Si oxide layer. Si wafer 1 as adevice substrate shown in FIG. 16A is anodized to prepare a substrateconstituted of nonporous Si layer 2 and porous Si layer 3 formed thereonas shown in FIG. 16B. Epitaxial layer 4 is formed on the surface ofporous Si layer 3 as shown in FIG. 16C. Separately, Si wafer 5 as asupporting substrate shown in FIG. 16D is provided, and the surface isoxidized to form a substrate constituted of monocrystalline Si layer 6and Si oxide layer 7 on the surface as shown in FIG. 16E. The substrate(2, 3, 4) of FIG. 16C is turned over, and is placed on the substrate (6,7) of FIG. 16E with epitaxial layer 4 and Si oxide layer 7 counterposedas shown in FIG. 17A. The two substrates are bonded by adhesion ofepitaxial layer 4 to Si oxide layer 7 as shown in FIG. 17B. Thennonporous Si layer 2 is removed mechanically by grinding from thenon-bonded layer side to uncover porous Si layer 3 as shown in FIG. 17C.Porous Si layer 3 is removed by wet etching with an etching solution forselective etching of porous Si layer 3 as shown in FIG. 17D to obtain aSOI substrate having epitaxial layer 4 of extremely uniform thicknessfor a semiconductor of an SOI substrate.

In the above process for producing the SOI substrate, nonporous Si layer2 is removed from the substrate of FIG. 17B by grinding to obtain thesubstrate of FIG. 17C. Therefore, one substrate 1 to be worked intotwo-layers of nonporous layer 2 and porous layer 3 is required forproduction of one SOI substrate. Japanese Patent Application Laid-OpenNo. 7-302889 discloses a method of repeated use of nonporous Si layer 2in the SOI substrate production process. In the disclosed process, parts4,7,6 for the SOI substrate are separated from part 2 at porous layer 3by applying a pulling, squeezing, or shearing force, or inserting a jiginto porous layer 3, and separated nonporous Si layer 2 is usedrepeatedly as Si wafer 1 of FIG. 16A.

Nowadays, most solar cells employ amorphous Si for the structuresuitable for large areas. However, monocrystalline Si andpolycrystalline Si are also noticed for the solar cells in view of thetransducing efficiency and the life thereof. Japanese Patent ApplicationLaid-Open No. 8-213645 discloses a process of providing a thin filmsolar cell at a low cost. In this process, porous Si layer 3 is formedon Si wafer 1; thereon, p⁺ -type Si layer 21, p-type Si layer 22, and n⁺-type Si layer 23 are grown epitaxially for solar cell layers;protection layer 30 is formed on n⁺ -layer 23; jig 31 is bonded to thereverse face of Si wafer 1 and jig 32 is bonded onto the surface ofprotection layer 30 by adhesive 34; jigs 31,32 are pulled respectivelyin opposite directions to break porous Si layer 3 mechanically toseparate solar layers 21,22,23. The solar cell layers 21,22,23 areinterposed between two plastic substrates to provide a flexible thinfilm solar cell. This disclosure mentions repeated use of Si wafer 1,and partial notching 33 of the edge side face of porous Si layer 3 by amechanical method or laser beam irradiation.

In production of SOI substrates, the process disclosed in theaforementioned Japanese Patent Application Laid-Open No. 7-302889enables reduction of the production cost by repeated use of the Siwafer. However, this method is not satisfactory in reproducibility.

In production of solar cells, the process disclosed in the aboveJapanese Patent Application Laid-Open No. 8-213645 does not always allowdefinite separation at the porous Si layer, causing occasional crackingin the epitaxial layer to result in lower production yield. Further,this process conducts the separation by mechanical pulling, whichrequires strong adhesion between the jig and the monocrystalline Silayer and is not suitable for mass production.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for forming athin film at a low cost, with definite separation of a wafer, withefficient utilization of the wafer, with effective use of the resourcewith high productivity in production of photoelectric transducer devicessuch as solar cells.

After comprehensive studies for solving the above problems, the processbelow has been found by the inventors of the present invention. Theprocess of the present invention for forming a thin film comprises astep of separation of a substrate constituted of a nonporous layer, aporous layer formed thereon, and a less porous layer which has a lessporosity than that of the porous layer formed further on the porouslayer into the nonporous layer and the less porous layer at the porouslayer, the step of separation being caused by projecting a laser beamthrough the side face of the substrate into the center of the substrate.

The laser beam is preferably focused onto the side edge face of theporous layer to swell the porous layer. The porous layer is formed onthe nonporous layer desirably by anodization of a Si wafer. Thenonporous layer may be pulled by applying a slight force to thesubstrate by a vacuum chuck brought into close contact with the facereverse to the porous layer side.

The laser beam is preferably an excimer laser beam in the presentinvention. The laser beam may be projected to plural sites of the porouslayer. The laser beam may be focused by a cylindrical lens linearly andis projected along the porous layer.

The aforementioned less porous layer is preferably formed on the porouslayer by epitaxial growth. After the epitaxial layer is bonded to asupporting substrate having an insulation layer at least on the surface,the separation is caused at the porous layer. The porous layer remainingon the epitaxial layer is removed to utilize the epitaxial layer and theinsulation layer respectively as a semiconductor layer and anunderlaying insulation layer of a SOI substrate. The supportingsubstrate having an insulating layer at least on the surface ispreferably prepared by oxidation of the surface of a Si wafer.Otherwise, an insulating layer is formed on the epitaxial layer surface;it is bonded to a supporting substrate; separation is caused at theporous layer; the remaining porous layer is removed from the epitaxiallayer; and the epitaxial layer and the insulating layer are employedrespectively as the semiconductor layer and the underlying insulationlayer of the SOI substrate. The supporting substrate may be either a Siwafer having an oxidized surface or a quartz substrate.

The less porous layer may be formed, after the formation of the porouslayer by anodization of the wafer, by subsequent anodization with lesscurrent density. After the bonding of the less porous layer to thesupporting substrate, the separation is caused, and the less porouslayer may be employed as a photoelectric transducing layer of aphotoelectric transducer. The photoelectric transducing layer may bemade of an epitaxial layer. The substrate and the layers are preferablyformed from silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a separation step in Embodiment 1 in which a laserbeam is projected to a porous Si layer.

FIG. 2 illustrates substrates after the separation.

FIG. 3 illustrates a step for producing an SOI substrate by employing aSi wafer as the base material in Embodiment 1.

FIG. 4 illustrates a SOI substrate produced by a step for producing aSOI substrate by employing a Si wafer as the base material in Embodiment1.

FIGS. 5A, 5B, 5C, and 5D illustrate steps for producing a SOI substrateby employing a quartz plate as the base material in Embodiment 1.

FIGS. 6A, 6B, 6C, and 6D illustrate steps for producing a SOI substrateby employing a Si wafer as the base material in Embodiment 1.

FIGS. 7A, 7B, 7C, and 7D illustrate steps for producing a SOI substrate.

FIGS. 8A, 8B, 8C, and 8D illustrate steps for producing a SOI substrate.

FIG. 9 illustrates a separation step in Embodiment 2 in which a laserbeam is projected to a porous Si layer.

FIG. 10 illustrates a separation step in Embodiment 3 in which a laserbeam is projected to a porous Si layer.

FIG. 11 illustrates a separation step in Embodiment 4 in which a laserbeam is projected to a porous Si layer.

FIGS. 12A, 12B and 12C illustrate processes for producing amonocrystalline Si solar cell.

FIG. 13 illustrates a separation step in Embodiment 5 in which a laserbeam is projected to a porous Si layer.

FIG. 14 illustrates substrates after the separation.

FIG. 15A is a perspective view of a monocrystalline Si solar cell, and

FIG. 15B is a sectional view thereof.

FIGS. 16A, 16B, 16C, 16D and 16E illustrate steps of producing a SOIsubstrate.

FIGS. 17A, 17B, 17C and 17D illustrate steps of producing a SOIsubstrate.

FIG. 18 illustrates a conventional process for producing a solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described by reference to Embodiments 1 to 8.Embodiments 1 to 4 describe production of a SOI substrate. Embodiments 5to 7 describe production of photoelectric transducing devices such assolar cells and area sensors. Embodiment 8 describes a method in whichthe layer for the separation is formed by ion implantation. The presentinvention covers not only the described Embodiments but also combinationof the Embodiments.

In Embodiment 1 of producing a SOI substrate, a laser beam is employedto separate the wafer to be recycled and the SOI substrate at the porouslayer. The laser beam is projected to the side face (edge face) inparallel with the plate-shaped substrates at a laser intensity adjustedto reach the substrate center. The laser beam is projected to arelatively fragile layer such as a larger porosity layer or a defectivelayer having micro-bubbles and is absorbed thereby. The porous fragilelayer having absorbed the laser beam becomes more fragile, whereby theless porous layer on the porous fragile layer and nonporous layer areseparated at the fragile layer. The method of laser beam projection isdescribed in detail in the following Embodiments.

Embodiment 1

FIG. 1 illustrates the separation process of this embodiment. In FIG. 1,the same reference numerals are used as in FIGS. 16A to 16E and FIGS.17A to 17D. The numeral 10 indicates a lens, 11 an optical microscope,12 a vacuum chuck, and 13 a laser beam. LS indicates a laser source, andATL indicates an article constituted of layers 2, 3, 4, 6, and 7 beforethe separation. A laser beam is projected to the side wall of porouslayer 3 from laser source LS. Laser source LS is an excimer laser ofhigh output employing XeCl, KrF, ArF, or the like. The output capacityis preferably in the range from 300 mJ/cm² to 1 J/cm², more preferablyabout 500 mJ/cm². The excimer laser emits UV light, so that lens 10 ismade of quartz or fluorite capable of transmitting the UV light. Withthis optical system, the laser beam is converged to have a projectionarea width to 0.1 μm. Optical microscope 11 is employed, as necessary,to confirm the precise projection of laser beam 13 to porous layer 3 of0.1 to 30 μm thick. Since porous layer 3 is fragile and can readily bebroken for separation in comparison with nonporous layer 2 and epitaxiallayer 4 as a less porous layer, laser beam 13 need not be strictlyprojected only to porous layer 3. The laser source LS for emitting laserbeam 13 is preferably an excimer laser apparatus of high power, but maybe an Ar laser, or a YAG laser. For promoting the layer separation, aliquid such as water, methyl alcohol, ethyl alcohol, and isopropylalcohol may be incorporated into porous layer 3 by injection oradsorption. The liquid, which has a larger thermal expansion coefficientthan a solid like Si, promotes the layer separation by expansion.

Vacuum chuck 12 as the substrate holder has a cavity for holding a gas,and holds substrate ATL, before the separation, by contact with theoutside face of nonporous layer 2 or monocrystalline layer 6 byevacuation of the gas. In this Embodiment, a pair of vacuum chucks 12can rotate around the axis at the center of the substrate to projectlaser beam 13 to the entire side wall of the porous layer 3. Vacuumchucks 12 may be employed only for fixing and rotating the substrate,but may also be employed for applying slight pulling force to thesubstrate to promote the layer separation.

The laser beam penetrates the side wall of porous layer 3 to theproximity to the center of substrate ATL. Porous layer 3 becomes morefragile by absorption of the laser beam to cause separation of substrateATL without breaking the nonporous portion.

By the separation, substrate 4,7,6 for the SOI substrate and substrate 2to be recycled are separated from each other at porous layer 3. In FIG.2, portions 3' of porous layers remain on the surfaces (separationplane) of the respective substrates. By reducing sufficiently thethickness of porous layer 3 formed by the anodization, the remainingportions can be practically removed from one or both of the substrates.

A process for thin film formation by the wafer separation as shown inFIG. 1 is described below.

First, the substrate for the bonding is prepared. FIG. 3 is a sectionalview of an apparatus for anodizing a Si wafer. The numeral 1 indicates aSi wafer, 27 an etching solution of hydrofluoric acid type contained invessel RV, 28 a positive metal electrode, and 29 a negative metalelectrode. Si wafer 1 to be anodized is preferably of a p-type, but maybe of an n-type of low resistance. An n-type wafer having holes formedby projection of a light beam can also be readily made porous. In FIG.3, a voltage is applied between positive electrode 28 at the left sideand negative electrode 29 at the right side, and Si wafer 1 is placedparallel to the both electrodes to apply an electric field perpendicularto the face of Si wafer in the etching solution. Thereby wafer 1 is madeporous from the side of negative electrode 29. Concentrated hydrofluoricacid (49% HF) is employed as an etching solution 27 of the hydrofluoricacid type. Since gas bubbles are generated from Si wafer 1, an alcoholis preferably added to the etching solution 27 as a surfactant to removethe bubbles. The alcohol may include methanol, ethanol, propanol, andisopropanol. Instead of surfactant addition, the solution may be stirredby a stirrer during the anodization.

The thickness of the porous layer is preferably in the range from 0.1 to30 μm.

Negative electrode 29 is preferably made of a material resistant againstthe hydrofluoric acid, such as gold (Au) and platinum (Pt). Positiveelectrode 28 is also made preferably of a material resistant against thehydrofluoric acid, but may be made of any generally used metal material.The anodization is conducted at a maximum current density of severalhundred mA/cm² or less, but should be more than 0 mA/cm². The currentdensity is selected so that a high-quality epitaxial layer can be formedon the resulting porous Si layer and the separation can readily becaused at the porous layer. Specifically, at a larger current density inthe anodization, the density of Si in the porous Si layer becomes lower,and the volume of the pores is larger, and the porosity (ratio of thepore volume to the total volume of the porous layer) thereof is larger.In spite of many pores formed in the interior of the Si layer, theresulting porous Si retains its monocrystallinity. Owing to themonocrystallinity of the porous Si layer, another monocrystalline Silayer can be grown epitaxially thereon.

For formation of an epitaxial Si layer having no lamination defects, theporosity of the porous Si layer is preferably smaller at the portion tobe brought into contact with the epitaxial Si layer. On the other hand,for easy separation of the device substrate and the SOI substrate at theporous Si layer, the porosity of the porous Si layer is preferablylarger. Therefore, ideally, the porosity of the porous Si layer issmaller at the outermost surface, and is larger at the side near to thenonporous Si layer. FIG. 4 is a sectional view illustrating the idealstate of the porous Si layer. Porous Si layer 3a, the surface sideportion of porous Si layer 3, is formed with a smaller porosity, andporous Si layer 3b, the nonporous Si layer side portion of porous Silayer 3, is formed with a larger porosity. For obtaining this structure,the initial stage of the anodization is conducted at a lower currentdensity to prepare portion 3a, and the later stage of the anodization isconducted at a higher current density to prepare portion 3b. With thisstructure, the separation of the substrate occurs only at layer 3b, andan epitaxial Si layer can be formed without lamination defects on porousSi layer 3a. The epitaxial Si layer is formed preferably by a growthprocess such as molecular beam epitaxial growth, plasma CVD, lowpressure CVD, photo-assisted CVD, bias sputtering, and liquid phasegrowth. Particularly, low temperature growth is preferred.

As described above, Si wafer 1 shown in FIG. 5A is made porous at thesurface as shown in FIG. 5B. Thereby Si wafer 1 comes to have thestructure constituted of nonporous Si layer 2 and porous Si layer 3laminated thereon.

Subsequently, nonporous epitaxial Si layer 4 is formed on porous Silayer 3 as shown in FIG. 5C. If necessary, the surface of epitaxial Silayer 4 is thermally oxidized to form Si oxide layer 8 with a thicknessbetween 0.05 μm to 2 μm as shown in FIG. 5D.

As described above, substrate PW called a prime wafer, a bond wafer, ora device substrate is treated before the bonding.

On the other hand, substrate HW called a handle wafer, a base wafer, ora supporting substrate is treated as described below.

A Si wafer is provided, and if necessary, the surface thereof isthermally oxidized to form a Si oxide film of thickness ranging from0.05 to 3 μm.

The process of bonding and separation of the substrates is explainedbelow by reference to FIGS. 6A to 6D.

The surface of Si oxide layer 8 on epitaxial Si layer 4 of substrate PWis confronted with the surface of Si oxide layer 7 of substrate HW, andboth faces are bonded at room temperature as shown in FIG. 6A. Then thebonding between Si oxide layer 8 and Si oxide layer 7 is strengthened byanode bonding, pressing, or heat treating, or combination thereof toform an article ATL comprising the bonded substrates as shown in FIG.6B.

Bonded article ATL having the structure shown in FIG. 6B is placedbetween a pair of vacuum chucks 12 of the device shown in FIG. 1. Withrotation of article ATL, an excimer laser beam is projected to andfocused on the portion of porous Si layer 3 of the side face of articleATL. The projected excimer laser beam is absorbed by the entire porousSi layer. In such a manner, nonporous Si layer 2 of substrate PW side isseparated from substrate HW as shown in FIG. 6C. By the separation,epitaxial Si layer 4 is transferred onto the surface of substrate HW.Porous Si layer 3 having been broken by absorption of the laser beam mayremain on one or both of nonporous Si layer 2 and epitaxial Si layer 4.FIG. 6C shows porous Si layer 3 remaining only on epitaxial Si layer 4.

Porous Si layer 3, when it remains on the side of substrate HW, isremoved selectively by selective etching. In the selective etching, theporous Si layer is etched more than the nonporous Si layer bynonelectrolytic wet chemical etching by use of an etching solution suchas hydrofluoric acid, a mixture of hydrofluoric acid with an alcohol,and a mixture of hydrofluoric acid with aqueous hydrogen peroxide. Inparticular, by use of a mixture of hydrofluoric acid with hydrogenperoxide, the selective etching ratio of the porous Si layer to thenonporous Si layer is as high as about 10⁵. Thus, epitaxial Si layer 4remains in a uniform thickness on the surface of substrate HW. Thereby aSOI substrate having extremely uniform semiconductor layer 4 on theinsulation layer as shown in FIG. 6D.

Separated nonporous layer 2 is again used as the prime wafer forproduction of another SOI substrate.

In the process for producing the SOI substrate in this Embodiment, thesupporting substrate may be a complete insulating substrate such as aglass substrate and a quartz substrate. FIGS. 7A to 7D illustrate thesteps of producing a SOI substrate by use of quartz substrate as thesupporting substrate. The device substrate PW at the upper side in FIG.7A is prepared in the same manner as described by reference to FIGS. 5Ato 5D. Quartz substrate 9 as supporting substrate HW is confronted witha Si oxide layer 8 and are bonded tightly by anode bonding, pressing, orthermal treatment, or combination thereof as shown in FIG. 7B. Then, thetwo substrates are separated by projection of a laser beam in the samemanner as described before. Epitaxial Si layer 4 and porous Si layer 3are transferred onto quartz substrate 9 as shown in FIG. 7C. Theremaining porous Si layer 3 is selectively removed in the aforementionedmanner. Thus a SOI substrate is obtained which is constituted of quartzbase plate 9 and nonporous monocrystalline thin Si film 4 formed thereonas shown in FIG. 7D.

In another process of preparing the SOI substrate of this Embodiment, aSi wafer is used as the supporting substrate, and an insulating layer ofa SOI structure is formed by forming a Si oxide layer on the epitaxialSi layer on the device substrate side without forming the Si oxide layeron the Si wafer side. FIGS. 8A to 8D shows this process. The upperdevice substrate in FIG. 8A is prepared in the same manner as describedby reference to FIG. 5. The surface of monocrystalline Si layer 5 of aSi wafer is counterposed with the surface of Si oxide layer 8, andbonded thereto. The bonding can be strengthened by anode bonding,pressing, or heating, or a combination thereof. Thus article ATL isobtained as shown in FIG. 8B. Article ATL is separated at the porous Silayer to transfer epitaxial Si layer 4 of nonporous monocrystalline Sionto nonporous Si layer 5 as supporting substrate HW by means of theapparatus shown in FIG. 1. If porous Si layer 3 remains on epitaxial Silayer 4 of supporting substrate HW as shown in FIG. 8C, it isselectively removed by the aforementioned method. Thus a SOI substrateis obtained as shown in FIG. 8D.

Embodiment 2

A SOI substrate is produced in this Embodiment by utilizing an excimerlaser beam for separating, at the porous Si layer, the Si wafer to berecycled and the substrate to be worked into a SOI substrate. Theexcimer laser beam is focused on one point and is allowed to scan withthe substrate plate fixed.

FIG. 9 illustrates the separation step. The laser beam is focused on theside face of article ATL with lens 10, and is allowed to scan along thecircumference by aid of guide 14. The same reference numbers are used toindicate the same articles as in FIG. 1. In this Embodiment,monocrystalline Si layer 6 as nonporous layer and nonporous Si layer 2constituting article ATL are fixed from the outside by chucks 12. Thelaser beam 13 from an excimer laser apparatus is focused and projectedonto one point on the side wall of porous Si layer 3 through lens 10.Lens 10 is allowed to move together with laser beam 13 for the scanningto separate the SOI substrate composed of layers 4, 7, 6 from substrate2 at porous Si layer 3 to recycle substrate 2 to the production steps.The less porous layer 4 on the layers 6, 7 is obtained. The other stepsand the materials are the same as in Embodiment 1.

Embodiment 3

A SOI substrate is produced in this Embodiment by utilizing an excimerlaser for separating the Si wafer to be recycled and the SOI substrateat the porous Si layer. The excimer laser beam is focused linearly by acylindrical lens and is projected along the side face of the porous Silayer.

FIG. 10 illustrates the separation step. The numeral 15 indicates acylindrical lens. The same reference numbers are used to indicate thesame articles as in FIG. 1. The laser beam is focused linearly in avertical direction for efficient projection onto the side face of porousSi layer 3 of an extremely small thickness of 0.1 to 30 μm. In place ofcylindrical lens 15, a toric lens may be used to project linearlyfocused laser beam to the curved side wall of porous Si layer 3. Theother steps are the same as in Embodiment 1.

Embodiment 4

A SOI substrate is produced in this Embodiment by utilizing an excimerlaser for separating, at the porous Si layer, the Si wafer to berecycled and the SOI substrate. The excimer laser beams are focusedlinearly by cylindrical lenses and are projected along the end face ofthe porous Si layer. In the laser beam projection, as shown in FIG. 11,the laser light is separated into four beams, and laser beams 13 arerespectively focused linearly and projected with four cylindrical lenses15 from four directions along the end face of porous Si layer 3. In thisEmbodiment, monocrystalline Si layer 6 and nonporous Si layer 2 arefixed from the outside by chucks 12. The other steps are the same as inEmbodiment 1.

Embodiment 5

A solar cell is produced in this Embodiment. FIGS. 12A to 12C illustratethe steps of forming a photoelectric transducing layer for transducinglight energy to electric energy. A p-type Si wafer 1 is provided asshown in FIG. 12A. The surface of Si wafer 1 is made porous byanodization as explained before by reference to FIG. 3 to prepare asubstrate constituted of nonporous Si layer 2 of wafer 1 and porous Silayer 3 formed thereon as shown in FIG. 12B. On porous Si layer 3, anepitaxial Si layer to serve as photoelectric transducer 18 is formed asshown in FIG. 12C by molecular beam epitaxial growth, plasma CVD, lowpressure CVD, photo-assisted CVD, bias sputtering, liquid crystal growthmethod, or a like method to prepare a substrate PW.

The epitaxial Si layer is allowed to grow with addition of a dopant foruse as a photoelectric transducing layer. Thereby, the epitaxial layerhas PN junction caused by lamination of an n⁺ layer, a p⁻ layer, and ap⁺ layer formed in this order on porous Si layer 3. The surface of thep⁺ layer of photoelectric transducing layer 18 grown epitaxially isbonded and connected to backside metal electrode 16 formed preliminarilyon the surface of plastic substrate 17. Then vacuum chuck 12 is broughtinto close contact with the outside of nonporous Si layer 2. A laserbeam 13 from an excimer laser apparatus is focused and projected throughlens 10 onto porous Si layer 3. Although the laser light is focused ontoone point in FIG. 13, the laser beam may be projected in any way asdescribed in Embodiments 1 to 4. In such a manner, substrate HW to beused as a solar cell is separated at porous Si layer 3 from substrate PWto be recycled to production process as shown in FIG. 14.

A meshed surface metal electrode 19 is formed on the surface ofphotoelectric transducing layer 18 as shown in FIG. 15A. Wiring 24 isconnected to surface metal electrode 19 and backside metal electrode 16.Protection layer 20 is formed on the surface metal electrode 19. FIG.15B is a sectional view taken at 15B--15B in FIG. 15A. Photoelectrictransducing layer 18 is constituted of the layers of n⁺ layer 23 incontact with surface metal electrode 19, p layer 22, and p⁺ layer 21 incontact with backside metal electrode 16, arranged in this order fromthe upper side. In FIGS. 15A and 15B, surface metal electrode 19 isshown in a shape of a mesh to transmit light. However, it may bereplaced by a transparent electrode made of a material like ITO.Backside metal electrode 16 serves also as a back reflector forreturning unabsorbed transmitted light to photoelectric transducinglayer 18, so that it is preferably made of a metallic material havinghigh reflectivity.

According to this Embodiment, many solar cells of a monocrystalline thinfilm can be produced from one Si wafer. Therefore, this Embodiment isadvantageous in transducing efficiency, cell life, and production cost.Further, the substrate is separated by projecting a laser beam to theporous Si layer to cause thermal expansion and distortion of the crystalwithout application of a strong pulling force and without necessity ofstrong bonding between the substrate and the jig or the like. From thisviewpoint also, the process of this Embodiment is advantageous inproduction cost.

Embodiment 6

A solar cell is produced in this Embodiment also. In the aboveEmbodiment 5, photoelectric transducer 18 is constituted of an epitaxialSi layer formed on porous Si layer 3, whereas in this Embodiment, porousSi layer of a small porosity per se is utilized, as it is, asphotoelectric transducing layer 18. In Embodiment 1, it is describedthat the porosity of the porous Si layer can be varied by changing thecurrent density in anodization. Specifically, in anodization explainedby reference to FIG. 3, the higher density of the electric currentflowing from electrode 28 to electrode 29 increases the porosity ofporous Si layer formed on Si wafer 1, while the lower density of thecurrent decreases the porosity. By this phenomenon, the surface of p⁺type Si wafer 1 is made porous by forming a porous Si layer of a smallerporosity by controlling the current density lower, and porous Si layer3b of larger porosity is formed under it on nonporous layer 2. To theoutermost layer of porous Si layer 3a, donor ions such as P and As areimplanted to make the outermost layer on n-type, thereby forming aphotoelectric transducing layer having a porous Si layer of smallporosity having a PN junction.

Thereafter, the porous Si layer of a smaller porosity as thephotoelectric transducer is bonded to backside metal electrode 16 asshown in FIG. 13. The other steps are conducted in the same manner as inEmbodiment 5.

According to this Embodiment, many solar cells of a monocrystalline thinfilm can be produced from one Si wafer. Therefore, this Embodiment isadvantageous in transducing efficiency, cell life, and production cost.Further, epitaxial growth is not conducted in the process of thisEmbodiment, and the production cost is lower than that of Embodiment 5.The photoelectric transducing layer 18 is composed of a porous Si layerof smaller porosity with the monocrystallinity maintained, and light isscattered appropriately by the pores to result in high transducingefficiency.

Embodiment 7

An area sensor is produced. In this Embodiment, a photoelectrictransducing layer of monocrystalline thin film is formed from an Siwafer in the same manner as in Embodiment 5 or 6. On this photoelectrictransducing layer, optical sensors are arranged two-dimensionally, andmatrix wiring is provided. The matrix wiring is formed, for example, byproviding a row wiring in place of surface metal electrode 19 in FIGS.15A and 15B and providing a line wiring in place of backside metalelectrode 16 in FIGS. 15A and 15B. This Embodiment is advantageous intransducing efficiency, cell life, production cost, area enlargement,and so forth, since many area sensors of a monocrystalline thin film canbe produced from one Si wafer.

Embodiment 8

An Si wafer is prepared as one substrate. Over the entire face of the Siwafer, hydrogen ions or rare gas ions are implanted in a prescribeddepth by means of an ion implanter to form a defective layer ofmicrobubbles in the interior of the Si wafer.

Separately, another Si wafer is prepared as the supporting substrate.This supporting substrate is oxidized at the surface, and is bonded tothe surface of the above Si wafer having a microbubble-defective layer.

To the bonded wafer article, an excimer laser beam is projected aroundthe microbubble-defective layer of the side face of the article in amanner shown in FIGS. 1, 9, 11, or 13. Thereby, the defective layer ismade more fragile by absorption of the excimer laser beam. Then the twowafers are separated.

In such a manner, the monocrystalline Si layer on the defective layer ofthe one substrate Si layer is transferred onto the silicon oxide film onthe other substrate. The microbubble formation by ion implantation isdescribed in detail in U.S. Pat. No. 5,374,564.

The above description is made regarding the case of Si wafers, but thepresent invention can be applied to other semiconductors other than Si,such as SiGe, Ge, SiC, GaAs, and InP.

According to the present invention, many monocrystalline thin Si filmscan readily be produced by projecting a laser beam from the side face ofa substrate into the center portion of the porous layer to causeabsorption of the laser beam by the porous layer. Since the laser beamdoes not cause impurity contamination, the resulting thin Si film hashigh quality, and the resulting SOI substrate also has high quality. Inproduction of SOI substrates, the material is used with less waste,resulting in low production cost and resource saving. The resultingphotoelectric transducing apparatus also has high quality. Also inproducing photoelectric transducing apparatus, the material is used withless waste, resulting in low production cost and resource saving.

What is claimed is:
 1. A process for forming a thin film comprising thesteps of:providing a substrate having:a nonporous layer; a porous firstlayer formed on the nonporous layer; and a second layer which is lessporous than the porous first layer; and separating the substrate byprojecting a laser beam through the side face of the substrate into thecenter of the substrate.
 2. The process for forming a thin filmaccording to claim 1, wherein the separating step further comprises astep of focusing the laser beam on the side face of the porous firstlayer to swell the porous first layer.
 3. The process for forming a thinfilm according to claim 1, wherein the providing step further comprisesa step of forming the porous first layer in the nonporous layer byanodizing a Si wafer.
 4. The process for forming a thin film accordingto claim 1, wherein the separating step further comprises a step ofprojecting the laser beam onto plural sites of the porous first layer.5. The process for forming a thin film according to claim 1, wherein theseparating step further comprises the steps of:bringing the face of thenonporous layer reverse to the porous first layer into close contactwith a vacuum chuck; and applying a slight pulling force by the vacuumchuck to the substrate.
 6. The process for forming a thin film accordingto claim 1, wherein the laser beam is an excimer laser beam.
 7. Theprocess for forming a thin film according to claim 1, wherein theseparating step further comprises the steps of:converging the laser beamlinearly by a cylindrical lens; and projecting the linearly convergedlaser beam along the porous first layer.
 8. The process for forming athin film according to claim 3, wherein the providing step furthercomprises a step of forming the second layer which is a nonporousepitaxial layer by epitaxial growth on the porous first layer.
 9. Theprocess for forming a thin film according to claim 8, the processfurther comprises the steps of:bonding the nonporous epitaxial layer toa supporting substrate with interposition of an insulating layer;separating the nonporous epitaxial layer at the porous first layer;removing the porous first layer remaining on the nonporous epitaxiallayer; and employing the nonporous epitaxial layer and the insulatinglayer respectively as a semiconductor layer and an underlying insulatinglayer of an SOI substrate.
 10. The process for forming a thin filmaccording to claim 9, the process further comprising the stepsof:providing a Si wafer having an oxidized surface as the supportingsubstrate; and employing the oxidized surface as the insulating layer.11. The process for forming a thin film according to claim 8, theprocess further comprising the steps of:forming an insulating layer onthe nonporous epitaxial layer; bonding the nonporous epitaxial layer toa supporting substrate; separating the nonporous epitaxial layer fromthe porous first layer; removing the porous first layer remaining on thenonporous epitaxial layer; and employing the nonporous epitaxial layerand the insulating layer respectively as a semiconductor layer and anunderlying insulating layer of an SOI substrate.
 12. The process forforming a thin film according to claim 11, wherein the supportingsubstrate is a quartz substrate.
 13. The process for forming a thin filmaccording to claim 11, the process further comprising a step ofpreparing the supporting substrate by oxidizing a surface of a Si wafer.14. The process for forming a thin film according to claim 3, whereinthe providing step further comprises a step of forming the second layerby anodizing the Si wafer at a current density lower than that used informing the porous first layer by anodizing.
 15. The process for forminga thin film according to claim 8 or 14, the process further comprisingthe steps of:bonding the second layer to a supporting substrate;separating the second layer at the porous first layer and employing thesecond layer as a photoelectric transducing layer of a photoelectrictransducing apparatus.
 16. The process for forming a thin film accordingto claim 1, wherein the providing step further comprises a step ofproviding a substrate having:the nonporous layer, the porous firstlayer, and the second layer,which are respectively composed of silicon.17. The process for forming a thin film according to claim 16, whereinthe separating step further comprises a step of focusing the laser beamon the side face of the porous first layer to swell the porous firstlayer.
 18. The process for forming a thin film according to claim 16,wherein the providing step further comprises a step of forming theporous first layer on the nonporous layer by anodizing a Si wafer. 19.The process for forming a thin film according to claim 16, wherein theseparating step further comprises a step of projecting the laser beamonto plural sites of the porous first layer.
 20. The process for forminga thin film according to claim 16, wherein the separating step furthercomprises the steps of:bringing the face of the nonporous layer reverseto the porous first layer into close contact with a vacuum chuck; andapplying a slight pulling force by the vacuum chuck to the substrate.21. The process for forming a thin film according to claim 16, whereinthe laser beam is an excimer laser beam.
 22. The process for forming athin film according to claim 16, wherein the separating step furthercomprises the steps of:converging the laser beam linearly by acylindrical lens; and protecting the linearly converged laser beam alongthe porous first layer.
 23. The process for forming a thin filmaccording to claim 18, wherein the providing step further comprises astep of forming the second layer which is a nonporous epitaxial layer byepitaxial growth on the porous first layer.
 24. The process for forminga thin film according to claim 23, the process further comprising thesteps of:bonding the nonporous epitaxial layer to a supporting substratehaving an insulating layer at least on the surface thereof; separatingthe nonporous epitaxial layer at the porous first layer; removing theporous first layer remaining on the nonporous epitaxial layer; andemploying the nonporous epitaxial layer and the insulating layerrespectively as a semiconductor layer and an underlying insulating layerof an SOI substrate.
 25. The process for forming a thin film accordingto claim 24, the process further comprising the steps of:providing a Siwafer having an oxidized surface as the supporting substrate having aninsulating layer at least on the surface thereof; and employing theoxidized surface as the insulating layer.
 26. The process for forming athin film according to claim 23, the process further comprising thesteps of:forming an insulating layer on the nonporous epitaxial layer;bonding the nonporous epitaxial layer to a supporting substrate;separating the nonporous epitaxial layer from the porous first layer;removing the porous first layer remaining on the nonporous epitaxiallayer; and employing the nonporous epitaxial layer and the insulatinglayer respectively as a semiconductor layer and an underlying insulatinglayer of an SOI substrate.
 27. The process for forming a thin filmaccording to claim 26, wherein the supporting substrate is a quartzsubstrate.
 28. The process for forming a thin film according to claim26, the process further comprising a step of preparing the supportingsubstrate by oxidizing a surface of a Si wafer.
 29. The process forforming a thin film according to claim 18, wherein the providing stepfurther comprises a step of forming the second layer by anodizing the Siwafer at a current density lower than that used in forming the porousfirst layer by anodizing.
 30. The process for forming a thin filmaccording to claim 18 or 29, the process further comprising the stepsof:bonding the second layer to a supporting substrate; separating thesecond layer from the porous first layer; and employing the second layeras a photoelectric transducing layer of a photoelectric transducingapparatus.
 31. The process for forming a thin film according to claim 1,wherein the providing step further comprises a step of forming theporous first layer by injecting hydrogen ions or ions of rare gas intothe substrate.
 32. A process for forming a thin film comprising thesteps of:forming a substrate having:a nonporous layer; a porous firstlayer formed on the nonporous layer; and a second layer which is lessporous than the porous first layer; and separating the second layer fromthe nonporous layer; wherein the separating step comprises a step ofprojecting a laser beam onto the side face of the substrate toward thecenter of the substrate.
 33. The process for forming a thin filmaccording to claim 32, wherein the separating step further comprises astep of focusing the laser beam on the side face of the porous firstlayer to swell the porous first layer.
 34. The process for forming athin film according to claim 32, wherein the forming step furthercomprises a step of forming the porous first layer on the nonporouslayer by anodizing a Si wafer.
 35. The process for forming a thin filmaccording to claim 32, wherein the separating step further comprises astep of projecting the laser beam onto plural sites of the porous firstlayer.
 36. The process for forming a thin film according to claim 32,wherein the separating step further comprises the steps of:bringing theface of the nonporous layer reverse to the porous first layer into closecontact with a vacuum chuck; and applying a slight pulling force by thevacuum chuck to the substrate.
 37. The process for forming a thin filmaccording to claim 32, wherein the laser beam is an excimer laser beam.38. The process for forming a thin film according to claim 32, whereinthe separating step further comprises the steps of:converging the laserbeam linearly by a cylindrical lens; and projecting the linearlyconverged laser beam along the porous first layer.
 39. The process forforming a thin film according to claim 32, wherein the second layer is anonporous layer.
 40. The process for forming a thin film according toclaim 32, the process further comprising the steps of:bonding the secondlayer to a supporting substrate with interposition of an insulatinglayer; separating the second layer from the porous first layer; removingthe porous first layer remaining on the second layer; and employing thesecond layer and the insulating layer respectively as a semiconductorlayer and an underlying insulating layer of an SOI substrate.
 41. Theprocess for forming a thin film according to claim 32, the processfurther comprising the steps of:forming an insulating layer on thesecond layer; bonding the second layer to a supporting substrate;separating the second layer from the porous first layer; removing theporous first layer remaining on the second layer; and employing thesecond layer and the insulating layer respectively as a semiconductorlayer and an underlying insulating layer of an SOI substrate.
 42. Theprocess for forming a thin film according to claim 40 or 41, wherein thesupporting substrate is a quartz substrate.
 43. The process for forminga thin film according to claim 40 or 41, the process further comprisinga step preparing the supporting substrate by oxidizing the surface of aSi wafer.
 44. The process for forming a thin film according to claim 32,wherein the forming step further comprises a step of forming the secondlayer by anodizing the Si wafer at a current density lower than thatused in forming the porous first layer by anodizing.
 45. The process forforming a thin film according to claim 40 or 41, the process furthercomprising the steps of:bonding the second layer to a supportingsubstrate; separating the second layer from the porous first layer; andemploying the second layer as a photoelectric transducing layer of aphotoelectric transducing apparatus.
 46. The process for forming a thinfilm according to claim 32, the process further comprising forming asubstrate having:the nonporous layer, the porous first layer, and thesecond layer,which are respectively composed of silicon.